This invention relates to well drilling equipment, and in particular to cable tool drilling apparatus. The invention involves a casing hammer assembly for a cable tool drilling apparatus, as well as related equipment and the cable tool drilling apparatus itself.
Cable tool drilling apparatus is well known. Typical cable tool drilling involves utilization of an impact drilling string extending through a length of casing pipe. After the impact operations have proceeded for a while, the drilling string is withdrawn from the casing pipe and a heavy annular hammer is clamped to the drill stem. The drill stem is then returned to the casing pipe, and the impact operation resumes with the annular hammer then being repeatedly raised and released as in the drilling phase to drive the casing pipe farther into the ground. Once the casing pipe has been driven to a desired further depth, the drilling string is again raised, the annular hammer is removed, and the impact drilling operation resumes. This process is then repeated many dozens of times in order to sink a well of a desired depth.
As can be appreciated from the nature of this kind of an operation, the impact drilling must be interrupted many times in order to drive the casing further into the ground. In fact, most of the time spent "drilling" is actually spent running the drilling apparatus into and out of the bore hole, not in the actual drilling process itself.
A further disadvantage of impact drilling is the fact that the drill bit is often well down the bore hole from the end of the casing pipe. The drilling operation then often causes excessive amounts of loose materials to cave into the bore hole, thereby requiring additional time and effort to remove the excessive amount of material that accumulates. Also, water is a necessary lubricant used in the drilling process, and often it is necessary for an operator to climb up the drilling mast in order to access the top of the casing pipe in order to inject water, therefore also consuming additional time as well as wear and tear on the operator.
In order to eliminate the constant shifting between impact drilling and sinking of the casing pipe, other apparatus has been developed to make the drilling process a more continuous operation. For example, U.S. Pat. No. 5,310,014 discloses a cable tool drilling apparatus which includes a separate driving ram which allows the drilling string to be passed through the ram and permits the ram to be utilized at the same time that the impact drilling process proceeds. While a significant improvement over the prior intermittent drilling operation, the apparatus of this patent has its own disadvantages. First, the ram is relatively long and must fit over the casing pipe. Thus, a different size of ram must be employed for every different diameter of casing pipe. Also, because of the relatively long length of the ram, the final casing pipe cannot be driven close the ground and must be cut at an acceptable height above the ground. In addition, because the ram is constantly driven up and down axially in relation to the casing pipe, there is the potential for damage of the pipe during the hammering operation, unless the ram is held fairly precisely in relation to the casing pipe. Also, in order to move the ram out of alignment with the casing pipe, it must be lifted to a significant height, and then shifted in some manner, such as by swinging the entire derrick assembly to one side or the other, a time consuming process.